In telecommunication systems, there are numerous applications in which it is preferred to transmit either analog or digital signals over an optical fiber instead of over an electrically conductive cable. Recently there has been a considerable development effort focused on improving the microwave performance of fiber-optic links used in telecommunication systems. A major component involved in this developmental effort is an optical modulator.
Optical modulators, sometimes referred to as electro-optic modulators, receive coherent light in the form of a laser beam commonly referred to as an optical carrier. The optical modulator modulates the optical carrier with an informational signal having a relatively high frequency in the upper megahertz range. The optical modulator is commonly provided in the form of a Mach-Zehnder interferometer, well known in the art, and described, for example, in U.S. Pat. Nos. 4,502,037; 5,109,441; 5,168,534; and 5,373,383; all herein incorporated by reference. The modulator serves as a waveguide and may be provided as an integrated optic ("IO") device fabricated from a semiconductor or insulator material in a manner more fully described in U.S. Pat. No. 5,168,534, as well as in the technical article "Linear Interferometric Waveguide Modulator for Electromagnetic-Field Detection" of C. H. Bulmer et al, published in OPTICS LETTERS, Vol. 5, No. 5, May, 1980, and are herein incorporated by reference. The modulator provides an optical output which is commonly demodulated by a photovoltaic detector. The operation of the modulator and its associated detector circuit have certain drawbacks. More particularly, the modulator generates unwanted harmonic components during its operation and the detector circuit has limitations with regard to its sensitivity.